1. Field of the Invention
The invention relates to an improvement in a process for the separation of hydrocarbons which utilizes the preferential adsorbency of a solid material, such as a molecular sieve, to perform the separation. The separation process may be further characterized as one in which a moving-ged of adsorbent is simulated by periodically advancing the points at which the feed, desorbent and effluent streams are communicated with a fixed bed of adsorbent. The invention specifically relates to the manner in which the effluent streams of the bed of adsorbent are fractionated in order to recover the desorbent and purify the product streams.
2. Description of the Prior Art
Separation processes using a solid adsorbent and in which the transfer points of the feed streams, desorbent streams and effluent streams are periodically advanced in a unidirectional manner to simulate the action of a moving-bed of adsorbent are known to the art. Typical processes are shown in United States Pat. Nos. 3,201,491 (Cl. 260-676) and 3,274,099 (Cl. 208-310). In U.S. Pat. No. 3,510,423, the operation of such a process is further described, including the use of a flow distributor to periodically change the lines carrying the various process streams associated with the adsorption column. This reference teaches fractionating both the raffinate-containing effluent stream and the extract-containing effluent stream to recover the desorbent for reuse in the adsorption column.
When the points at which the raffinate-containing and the extract-containing effluent streams are removed from the column are changed, the composition of these streams takes a sudden step-like change toward a higher desorbent content. This sudden increase in what is normally the more volatile of the two components separated in the fractionation zone would cause an upset in the operation of the fractionation zone if uncorrected. To counteract this problem, the prior art has placed an accumulation zone in the effluent flow path between the flow distributor and the fractionation zone. The backmixing in this zone produces a uniform charge stock for the fractionation zone.
It is known in the art of fractionation that to obtain the best possible separation in an existing fractionation column, it is necessary to introduce the feed stream at a point determined by the condition of the feed stream and the composition of the liquid on different trays in the column. See for instance page 13-39 of The Chemical Engineer's Handbook, John H. Perry Ed., 4 Ed., McGraw-Hill, 1963.